1. Field of the Invention
The present invention relates to aqueous coating compositions, and more particularly to aqueous coating compositions in the form of an aqueous dispersion of acrylic copolymer prepared by polymerizing a mixture of acrylic monomers in an aqueous medium containing an electroconductive copolymer.
2. Description of the Prior Art
In recent years, various acrylic copolymer compositions have been proposed for coating plastics films (hereinafter referred to briefly as "films") with use of an aqueous medium to impart heat seal properties to the films as disclosed, for example, in U.S. Pat. No. 3,753,769 and Japanese Patent Publication No. 24223/1970. We have already invented coating compositions comprising an aqueous dispersion containing a specified acrylic copolymer for giving outstanding heat seal properties and filed a patent application therefor (Japanese Patent Application No. 73498/1976).
These coating compositions, which contain an aqueous medium, are usable free of the problem of residual solvent in the coatings formed from other known compositions containing an organic solvent. It is also known that the films coated with such coating compositions have transparency, stability against light and good heat seal properties and are useful as wrapping materials.
However, since the coated films have no anti-static properties, they become electrostatically charged easily when subjected to frictional contact with other articles, so that when used on an automatic machine for continuous packaging operation, the film is not smoothly automatically feedable, resulting in a seriously reduced efficiency. The surface of the film covering the wrapped or packaged articles is prone to dust deposition or staining, which greatly impairs the commercial value of the articles. To eliminate these problems due to electrostatic charges, it is desired to impart antistatic properties to the coated film.
In order to give films heat seal properties and antistatic properties, it is practiced to coat the film with a resin composition having heat seal properties and to apply an antistatic agent to the coating, but this method is not economical since it requires at least two coating steps. The method has another drawback that the film loses the antistatic properties when washed with water or subjected to friction because the antistatic agent is then easily removable. These drawbacks may be overcome by coating the film with a resin composition having heat seal properties and incorporating an antistatic agent. In fact, it has been attempted to practice this method with use of an electroconductive low-molecular-weight surfactant which is generally used as an antistatic agent so as to give the film heat seal properties and antistatic properties. However, the film, if given sufficient heat seal properties, fails to have high anti-static properties suited for recent high-speed packaging machines, whereas when given sufficient antistatic properties, the film is unable to have heat seal properties. Thus the method, unlike the present invention, involves difficulties in affording both heat seal properties and antistatic properties.